The contractor will establish and maintain a repository for biological and environmental samples collected from children and their families participating in the National Children?s Study (NCS). The NCS is a major longitudinal cohort study that will follow children from before birth to at least 21 years of age and will evaluate the effects of a variety of environmental factors on children?s health and well-being. Researchers will analyze how these factors interact with each other and what helpful and/or harmful effects they might have on children?s health.